One part of the PostgreSQL core that you may not necessarily have installed is what's called the contrib modules (it is named after the directory they are stored in). These are optional utilities shipped with the standard package, but that aren't necessarily installed by default on your system. The contrib code is maintained and distributed as part of the PostgreSQL core, but not required for the server to operate.
From a code quality perspective, the contrib modules aren't held to quite as high a standard, primarily by how they're tested. The main server includes heavy regression tests for every feature, run across a large build farm of systems that look for errors and look for greater performance and greater stability. The optional contrib modules don't get that same level of testing coverage. However, the code itself is maintained by the same development team, and some of the modules are extremely popular and well tested by users.
A list of all the contrib modules available can be found at at http://www.postgresql.org/docs/current/static/contrib.html.
